The principal object of the present invention is novel organic lens molds constituted wholly or in part of at least one specific inorganic glass. Said specific inorganic glass, when novel per se, constitutes a further object of the present invention.
The organic lenses are conventionally obtained by radical polymerization of a polymerizable composition cast between two complementary parts of a glass mold. At least one of these two parts has an internal surface which has an optical quality that confers an adequate surface quality to the cast lens.
Heretofore, the glasses used for constituting said organic lens molds have not in general been developed particularly to this end. Insofar as traditionally, the manufacturers of organic lenses are or have been glass lens manufacturers, the materials used for said molds for said organic lenses are in commercially available ophthalmic glasses. Such glasses have generally been treated thermally or chemically in order to strengthen their mechanical properties.
The inventors have summarized prior art inorganic glass compositions in columns 1 to 10 of the Table below. In double column 11 of said Table, the (novel and not novel) compositions of the glasses for the lens molds according to the invention are found, the composition that shall be reverted to in more detail a little further on in the text. The originality of the invention is demonstrated in considering said Table 1 and the comments relating thereto below.
The ophthalmic glasses described in U.S. Pat. No. 3,790,260 (Ref. 1) are glasses of high strength, strong UV absorption (insofar as TiO2 is a major constituent of it). They are free from ZrO2 and CaO.
The U.S. Pat. No. 4,042,405 (Ref. 2) describes a phosphosilicate glass having a compression layer depth greater than 120 xcexcm.
The U.S. Pat. No. 4,036,623 (Ref. 3) describes white glasses known as xe2x80x9ccrownxe2x80x9d glasses which are strengthened by a specific treatment which includes:
a pre-heating of said glasses,
then, an immersion of these glasses into a molten KNO3 bath at a temperature higher than the strain point of said glasses (advantageously at a temperature between 510 and 710xc2x0 C.); this without inducing optical distortion.
Said white glasses do not contain any B2O3 and generally comprise CaO.
The U.S. Pat. No. 4,015,045 (Ref. 4) describes glass compositions which are perfectly appropriate for producing flat glasses. Said compositions contain a significant amount of TiO2 and do not contain any ZnO.
The U.S. Pat. No. 4,012,131 (Ref. 5) describes a glass for ophthalmic lenses, strengthened by an ion exchange technique, which has a compression layer depth greater than 60 xcexcm. Said glass does not contain any ZnO.
The U.S. Pat. No. 4,259,118 (Ref. 6) describes thermally pre-stressable glasses which have a low linear coefficient of thermal expansion and high strain point and softening point. Said glasses do not contain any K2O.
The EP application EP-A-0 600 302 (Ref. 7) describes fast strengthenable boroaluminosilicate glass lenses (strengthenable in less than 4 hours). Said glass contains a large amount of B2O3, a small amount of ZnO (advantageously, it does not contain any ZnO) and can be ZrO2-free.
The German application DE-A-4 325 656 (Ref. 8) describes glass fire protection articles in a glass which is very rich in silica. No mention is made in this document of the applications of said glass in the optical and/or ophthalmic molds or lens fields.
Finally, the GB application GB-A-2 299 991 (Ref. 9) describes a chemically strengthened aluminosilicate glass for magnetic disks of low thickness. Said glass does not contain any B2O3, it contains Na2O at a maximal amount of 7% and the minimal amount of intervening MgO+CaO+BaO+SrO is more than 5%.
The use of these prior art inorganic glasses, which are different from those of the invention, has never been described nor suggested as molds for organic lenses.
The U.S. Pat. No. 3,951,671 (Ref. 10) describes glass compositions for use in making ophthalmic lenses or lens blanks which can subsequently be toughened by an ion-exchange process. The use of said glass compositions as molds for organic lenses is neither described nor suggested.
Within the particular context of molds for organic lenses, CORNING, SCHOTT and HOYA have especially used glasses known under the code-names:
QE-8092 (Danville), LJ-8361 (Danville), TRC 33 (Bagneaux) and BL, for CORNING (the first of these 4 glasses, referenced QE-8092, described in the U.S. Pat. No. 3,790,260, was placed on the market in the 1970""s. It was created in order to have a better impact resistance than the second of said 4 glasses, referenced LJ-8361, itself placed on the market in the 1940""s. To this end, said glass QE-8092 undergoes a chemical strengthening);
CHW-0991 and S-3 for SHOTT;
N-4 for HOYA.
Amongst these 7 glasses, only that referenced CHW-0991 is not a standard ophthalmic glass but is a glass developed specifically for preparing organic lens molds. Its properties are nevertheless very close to those of such a standard ophthalmic glass: QE-8092. The compositions of said two glassesxe2x80x94CHW-0991 and QE-8092xe2x80x94are in fact very close. Said two glasses contain significant amounts of TiO2 (about 0.8%).
The same applies for the two other glasses referenced TRC 33 and S-3. They contain 1.5 and 0.6% TiO2 respectively.
The LJ-8361 glass itself contains 0.4% TiO2 and 8.4% CaO.
The N-4 glass itself has a high Al2O3 content (14% by weight), Li2O (4.1% by weight), CaO (2.6% by weight) and does not contain any ZnO.
The BL glass the standard xe2x80x9ccrownxe2x80x9d white glass itself contains about 70% SiO2, 9% CaO and only 0.7% Al2O3. This glass is thermostrengthenable.
Within the context of inorganic glasses in general and inorganic glasses used for organic lens molds in particular, the inventors preside novel organic lens molds constituted wholly or in part of at least one specific inorganic glass which is free or almost free from TiO2 and which is perfectly suitable as a material for such organic lens molds. Said novel organic lens molds constitute the principal object of the present invention. The specific inorganic glasses likely to enter into their composition constitute a further object of the present invention when they are novel.
The organic lens molds of the invention are characteristically constituted wholly or in part of at least one inorganic glass having the following composition, expressed in percentages by weight:
(The + in the expression As2O3+Sb2O3 must be read: and/or).
According to an advantageous variant, said inorganic glasses are TiO2-free. Within the context of this advantageous variant, the particularly preferred glass compositions for the lens molds of the invention are given below:
These glasses (constitutive of the lens molds according to the invention) whose interesting properties are developed further on in the present text (it may already be mentioned that they are transparent white glasses which have a good UV transmission, a low sensitivity to said UVs, an interesting chemical durability and which are (easily) mechanically strengthenable) may advantageously be strengthened mechanically by methods of chemical or thermal tempering. Such methods are methods known per se.
A chemical tempering, advantageously carried out under specific conditions (in any case different from those described in the U.S. Pat. No. 4,036,623), is most particularly recommended for the reinforcement of these glasses.
During the method referred to as thermal tempering, the glass is, in a manner known per se, heated above its annealing point (typically, for the glasses in question, at temperatures corresponding to 1010.2-109.2 poises) and is then abruptly cooled by jets of air. The low thermal conductivity of the glass makes it that the core layers congeal and retract after the shallow layers, thus placing the latter layers under compression.
The profile of constraints obtained is parabolic with a core extension equal to half the surface compressions. The level of constraints is proportional to the strengthenability and the thickness of the glass.
The strengthenability of the glass is itself proportional to the CTE (dilation coefficient) of the glass and to its Young""s modulus and is inversely proportional to the thermal conductivity of the glass.
The glasses constitutive of the lens molds according to the invention are particularly suited to the thermal tempering (by virtue of their high CTEs and Young""s moduli, as well as by virtue of their softening point lower than 810xc2x0 C.: see later on): they can thus be treated within the traditional temperature ranges.
The chemical tempering allows mechanically strengthening a glass, for example, a glass lens, by compressing the surface of said glass. This result is obtained by an ion exchange mechanism; the glass being immersed in a molten salt bath at a given temperature. Under the effect of the temperature, an exchange takes place between the alkali ions (Na+, Li+), which leave the surface of the glass, and those which are larger (generally K+) which are present in the molten salt, which then penetrate the glass. After cooling, the surface of the treated glass is placed under compression compared to the core of said glass, this thus induces a strengthening of said glass by an increase of its resistance to breakage. The compressed layer thus formed is uniform.
According to the prior art, in the case of the xe2x80x9ccrownxe2x80x9d white glasses and fixed tint lenses, the chemical tempering is carried out for sixteen hours in a bath composed of 99.5% potassium nitrate (KNO3) and 0.5% silicic acid (H2SiO3), at a temperature of 450xc2x0 C.
The glasses constitutive of the lens molds according to the invention are therefore advantageously strengthened by such a chemical tempering method known per se. Within the context of the present invention, the inventors have most particularly adapted such a chemical tempering method to the specific glasses constitutive of the lens molds according to the invention. The inventors recommend therefore, according to a particularly advantageous variant, to carry out the chemical tempering with these specific glasses under the conditions below:
in a potassium or (and) sodium nitrate bath; and in a particularly preferred manner, in a potassium nitrate bath;
at a temperature between 425 and 475xc2x0 C.; and in a particularly preferred manner, at a temperature between 440 and 450xc2x0 C.;
for 10 to 20 hours; and in a particularly preferred manner for 12 to 20 hours (typically for 16 hours).
The thus thermally or chemically (advantageously chemically) treated glasses can have a shallow compression layer of depth greater than or equal to 70 xcexcm. Such glasses, with such a compression layer are particularly preferred. They have a mechanical resistance which is comparable, even better than that of prior art glasses known under the code name TRC 33 (mechanical resistance evaluated by MOR on unabrased samples).
It is now suggested to revert back in greater detail to the weight composition of the glasses constitutive of the lens molds according to the invention; the original composition which especially allows said glasses to respond as favorably to the thermal and chemical tempering methods.
Silica, SiO2, is the oxide which forms the network of the glass and it intervenes between 56 and 66% by weight in the composition of these glasses. If it intervenes in too low an amount ( less than 56%), the glass becomes susceptible to deterioration; if it intervenes in too great an amount ( greater than 66%), the glass becomes difficult to melt. The silica content of these glasses is advantageously between 61.5 and 63% by weight.
It is specified here in a general manner that the preferred ranges, indicated for the content of each one of the constituents are on the one hand preferred, in themselves, i.e. independent of the preferred ranges indicated for the other constituents and on the other hand, particularly preferred, taken in combination with said preferred ranges for the other constituents.
Al2O3 enables:
improving the chemical resistance of these glasses;
increasing the ionic exchange kinetics which takes place between an alkali of the shallow layer of said glasses and an alkali of a higher ionic level, and thus enables increasing the depth of the compression layer generated during the thermal or chemical tempering. Al2O3 intervenes between 2.5% and 10% by weight, above which value the glass becomes very viscous and difficult to melt. Preferably, the glasses constitutive of the lens molds according to the present invention have an Al2O3 content between 2.5 and 4%.
B2O3 enables improving the melting of these glasses. Said glasses contain between 0.5 and 7% thereof by weight. An excessive B2O3 content is detrimental to the durability of the glass as well as for the maintenance of the strain point above 495xc2x0 C. Preferably, the B2O3 content is between 0.5 and 3% by weight.
The alkali oxides act as flux during the melting of the glass, and they are therefore indispensable components for preparing said glass. Na2O is an essential component for the ionic exchange which takes place during the chemical tempering advantageously carried out. Its content is between 8 and 15% by weight in the glasses. If its content is too high, the chemical deteriorability appears and the strain point decreases.
The presence of K2O in the composition of the glasses enables improving the ionic exchange rate and controlling the thickness of the compression layer by altering the proportion between the two oxides Na2O and K2O. Moreover, its introduction in the composition of the glasses instead of Na2O enables decreasing the corrosion of the surface after chemical tempering. K2O is present between 3 and 12% by weight and preferably between 5 and 10%.
Li2O may also be present in the composition, and this so as to increase the level of compression in the layer by exchange between the lithium and the potassium of the chemical tempering bath. The Li2O content is between 0 and 3% by weight and preferably between 0 and 0.5% by weight.
The total alkali content (Li2O+Na2O+K2O) in the composition of said glasses is maintained between 12 and 20% by weight especially so as to control the level of compression, the depth of the compression layer and the durability before and after chemical tempering.
ZnO enables improving the melting of the glasses as well as their viscosity, this without interfering with the eventual chemical tempering carried out. ZnO is present at the rate of 2 to 12% by weight, advantageously at the rate of 7 to 12% by weight.
TiO2 may intervene in the composition of the glasses especially so as to improve their chemical durability. However, its presence at a content greater than 0.5% by weight induces an absorption in the UV between 310 nm and 400 nm, which is detrimental to the recommended use of said glasses (detrimental to the UV exposure treatment of the organic lenses which takes place across the glass molds of the invention). TiO2 is therefore always present in a content lower than or equal to 0.5%. Advantageously, it is excluded from the composition of the glasses constitutive of the lens molds according to the invention.
ZrO2 is an oxide which enables improving the chemical durability of the glasses and especially their alkali durability and their hydrolytic resistance. A minimum of 1% by weight is necessary for taking advantage of this effect in the glasses. If the ZrO) content is too great, the melting of the glass becomes very difficult. The ZrO2 content is therefore lower than or equal to 9%, and is preferably between 1 and 7% by weight.
The alkaline-earth elements, CaO, MgO, SrO, BaO have act as flux in an analogous way to the alkalis; this is the reason why they may advantageously be present in the glasses, so as to improve the melting and the forming of said glasses. But, insofar as CaO has a tendency to deteriorate the chemical strengthenability, its content remains between 0 and 1% by weight. The MgO content is between 0 and 3% by weight and those of BaO and SrO between 0 and 2% by weight.
The total CaO+MgO+BaO+SrO is between 0 and 5% by weight.
Cl is optionally present to improve the melting of the glass and to contribute to its finishing in contents between 0 and 0.5% by weight as well as other finishing agents, As2O3 and/or Sb2O3 which are themselves optionally present at a total content between 0 and 1% by weight. The intervention of other finishing agents (such as Br, F and/or SO3, for example) is in no way excluded from the context of the present invention.
To all useful ends, it is hereby specified that, for what relates to the optional components (Li2O, MgO, TiO2, CaO, BaO, SrO, Cl, As2O3, Sb2O3), the minimal intervening amount from which they exert a significant effect is generally in the order of 0.5%. Thus, the glasses constitutive of the lens molds according to the invention cannot contain said constituents or, if they contain them, it is generally in a minimal amount of 0.5% (% by weight).
The said glasses essentially consist of the constituents indicated above. It would not however be totally excluded that they contain other constituents within them. Such constituents can in any case intervene in low amounts and have not a significant influence on the properties sought after.
The said glasses, such as described above, are, as already indicated, transparent white glasses. Their properties are specified below. They are characterized:
by a good transmission in the UVs. Thus, their transmission at 315 nm is better than that of the BL glass, the reference in this field;
by a lower sensitivity to solarization under UV. The inventors have tested the said glasses in keeping them exposed for 200 hours under a Xenon lamp or a Mercury/Xenon lamp. In both cases, the inventors have been able to verify, by measuring the transmission, before and after exposure, that said glasses were not darkened. This is particularly important from the point of view of the application of said glasses sought after. In fact, the UV crosslinking of polymerizable compositions cast in glass molds (through the walls of said molds) is the technology which is carried out more and more for the production of organic lenses (by polymerization);
by the physical properties below:
strain point;  greater than 495xc2x0 C.,
softening point:  less than 810xc2x0 C.;
CTE: 80-95xc3x9710xe2x88x927/xc2x0 C.;
density:  less than 2.8;
Young""s Modulus:  greater than 70,000 Mpa;
Liquidus viscosity:  greater than 103 Paxc2x7s (10,000 poises).
xe2x80x83(Such Liquidus viscosity values are particularly interesting insofar as they make it possible to make the glasses by the standard industrial technologies);
by a very good chemical durability, which is greater than that of the lo prior art glasses known under the code names QE-8092 and TRC 33 (Corning glasses used for organic lens molds, vide supra) and comparable to that of the standard white xe2x80x9ccrownxe2x80x9d BL glass (also vide supra) which itself also can be thermally strengthened. Said chemical durability has been measured. The results below have been obtained for the said glasses:
acid durability (evaluated according to the standard DIN 12116 (vide infra)): on glasses tempered chemically in a KNO3 bath at 440xc2x0 C. for 16 hours, the weight loss is less than 3 mg/dm2; it is 1 mg/dm2 for certain preferred glasses;
alkali durability (evaluated according to the standard NF B35602 (vide infra)): on glasses tempered chemically (under the same conditions), the weight loss is less than 200 mg/dm2;
hydrolytic resistance (evaluated according to the standard NF B25601 (vide infra)): the weight loss is less than 150 mg/dm2;
by their thermal or chemical strengthenability: this has already been developed earlier on in the present text.
The manufacture of these glasses constitutive of the lens molds according to the invention does not present any particular difficulty; it does not require any unusual conditions or measures. The manufacture is within the reach of the person skilled in the art.
The classical starting materials, such as oxides, carbonates and nitrates, can be used for preparing fillers to be melted. The usual precautions, as for the purity of said intervening starting materials, in order to obtain optical glasses suffice (obviously if it is desired to obtain glasses of optical quality).